Fuel cells - the quest for renewable energy
TWI Bulletin, July/August 2001
Caroline Williams joined TWI as a project leader in September 2000. Previously she studied for a BSc in Chemistry and Biology at Nottingham Trent University, followed by an MPhil and PhD at Brunel University. Caroline has several years research experience in solid oxide fuel cells, particularly in materials selection and fabrication of fuel cell components, and more recently in joining and sealing of ceramic fuel cell systems. She is currently involved in the development of sol gel coating technology and high temperature materials for applications in the power generation sector.
John Fernie studied for a BSc in Metallurgy and Microstructural Engineering, followed by a PhD in Physics. He has more than 15 years' experience in ceramics and ceramic joining. Since 1999, John has been Manager of the Ceramics, Electronics and Microtechnology Group. Currently, he is engaged in ceramic-based innovations for the electronic, automotive and power generation sectors.
With the Kyoto Protocol requiring developed countries to reduce their greenhouse gas emissions by about 5% below their 1990s levels by 2012, renewable energies will play an important part. As Caroline Williams and John Fernie report renewable energy, the term used to describe a wide range of natural energy sources, is hoped to provide the planet with a more promising energy future.
It is estimated that approximately 15-20% of global energy needs are currently met by renewables such as biomass, hydroelectricity and geo-thermal, and the expectation is that this will significantly increase. Additional sources such as wind and solar power have received much attention but have failed to materialise on a global scale. Most recently the fuel cell has come to the fore. Essentially a fuel cell can be considered as a cross between an everlasting battery and an engine, in which electrical output is maintained as long as a fuel (hydrogen or hydrogen carrier) is supplied. Like a battery, it produces electricity by electrochemical means. Major initiatives are under way in the power generation and automotive industries; and on a smaller physical scale, there is increasing interest from the consumer and electronics industries.
Compared with conventional methods of energy generation, fuel cells offer several advantages; particularly higher conversion efficiency (50-60%) and far better emissions rate. The lowering of greenhouse emissions is now a real aim. SO x and NO x emissions are reported to be barely detectable, in contrast with gas turbines which generate much higher levels. Carbon dioxide emissions are still present, but they are approximately half those of gas turbines.
How a fuel cell works
Fuel cells are a fundamentally different way of producing electrical power from a variety of fuels. They convert the chemical energy of fuel directly to electrical energy without combustion.
A fuel cell consists of two electrodes (the anode and the cathode) separated by an electrolyte. Figure 1 shows a common type of fuel cell: a solid oxide fuel cell. In this type of fuel cell two processes occur simultaneously:
- Oxygen (air) is fed into the cathode where it is reduced (by accepting electrons from the external circuit) at the boundary between the cathode and electrolyte. The oxygen ions (O 2-) which are formed migrate through the electrolyte to the anode.
- Fuel, such as hydrogen, is fed into the anode where it is oxidised by the O 2- ions, which have migrated through the electrolyte. The O 2- ions combine with the hydrogen and electrons are released to the external circuit to complete the electrical loop. This reaction in turn produces water, the only waste product.
Fig.1. Operating principle of a fuel cell
Individual fuel cells typically generate a DC voltage of about 0.7-0.8V and power outputs of ten to hundreds of Watts. Therefore, practical fuel cells are not operated as single units but are connected in electrical series or parallel to build voltage; a series of cells is referred to as a stack.
Types of fuel cell
The fuel cell is in principle very simple; but, the chemical reactions do not readily take place. Unless specific materials are used for the electrodes and electrolytes, the current produced per cm 2 is extremely small and the electrical power losses in the electrolyte are very large. To overcome these problems, different types of fuel cells have been developed. Electrolyte, charge carrier and direction of charge flow are used to distinguish the different types as outlined in the Table.
Table: Main types of fuel cell
| Fuel cell type | Electrolyte (charge/carrier) | Direction of charge flow | Operating temperature, °C | Applications | Efficiency, % |
| PAFC | Phosphoric acid (H +) | Anode to cathode | 190-210 | Power generation | 35-45 |
| SPFC | Sulphonic acid incorporated in a solid polymer membrane (H +) | Anode to cathode | 60-90 | Automotive, power generation and consumer | 3-40 |
| MCFC | Molten mixture of lithium and potassium carbonates (CO 3 2-) | Cathode to anode | 550-650 | Power generation | 5-60 |
| SOFC | Solid ceramic, usually doped-zirconia or doped-ceria (O 2-) | Cathode to anode | 600-1000 | Power generation | 60-70 |
| AFC | Solution of potassium hydroxide | Cathode to anode | 70-90 | Automotive and aerospace | 55-60 |
| DMFC | Sulphonic acid, incorporated in a solid polymer membrane (H +) | Anode to cathode | 60-80 | Automotive | 35-40 |
| PAFC - phosphoric acid fuel cell; SPFC - solid polymer fuel cell; MCFC - molten carbonate fuel cell; SOFC - solid oxide fuel cell; AFC - alkaline fuel cell; DMFC - direct methanol fuel cell |
Phosphoric acid fuel cells (PAFC) are the most commercially developed type of fuel cell. Stacks of up to 200kW units are in commercial production and can achieve the moderate current densities most suitable for small-scale power plants. They are currently being used in hospitals, nursing homes, office buildings, schools, utility power plants and an airport terminal. PAFCs operate at temperatures between 190-210°C and generate electricity at more than 40% efficiency and nearly 85% efficiency if the steam produced by this fuel cell is used for co-generation with negligible emissions. This is compared to 30% efficiency for the most efficient internal combustion engine.
Solid polymer fuel cells (SPFC) operate at relatively low temperatures (60-90°C), have high power density and can vary their output to meet shifts in power demand. It is possible to combine the anode, cathode and membrane electrolyte in a compact unit and systems of <500kW are readily available. They are suited for applications in the transport sector where quick start-up is required. They are primary candidates for light-duty vehicles, for buildings and potentially for much smaller applications, such as replacements for rechargeable batteries.
Molten carbonate fuel cells (MCFC) promise high efficiencies and the ability to consume coal-based fuels. They have an operating temperature in the range of 550-650°C. MCFCs are mainly used in medium to large scale stationary power and combined heat and power systems, with outputs up to 1-2MW.
Solid oxide fuel cells (SOFC) are highly promising as they can be used in conjunction with large high-power applications. Some developers predict SOFCs could be used to power motor vehicles. Both MCFC and SOFC systems operate at relatively high temperatures and are most promising for stationary power generation. The SOFC is suited to integration with gas turbines and system studies have shown electrical efficiencies of approximately 70% are feasible.
NASA has long used alkaline fuel cells (AFC) on space missions. These cells can achieve power-generating efficiencies of up to 70%, although present performance restricts usage to low-power battery charging. They use alkaline potassium hydroxide as the electrolyte and have an operating temperature of 70-90°C. Until recently they were too costly for commercial applications, but several companies are examining ways to reduce costs and improve operating flexibility.
Direct methanol fuel cells (DMFC) are relatively new. These cells are similar to SPFC in that they both use a polymer membrane as the electrolyte. However in the DMFC, the anode catalyst itself draws the hydrogen from liquid methanol. Efficiencies of about 40% are expected with this type of fuel cell, which would typically operate at elevated temperatures.
Fuel cell design
There are three common stack configurations: planar, tubular and monolithic. These differ in the manner of sealing between fuel and oxidant channels and making cell to cell electrical connections.
In the simplest planar design, Fig.2, the components are configured as thin flat plates. The interconnections have ribs on both sides to form gas flow channels, connecting the anode and the cathode of adjoining cells. The planar cell design offers improved power density but requires high temperature seals to isolate the oxidant from the fuel. Difficulties in successfully developing such high temperature seals have limited the development and use of planar design cells.
The best progress has been achieved with tubular geometry cells, being developed in the USA and Japan. In this design ( Fig.3) the active cell components are deposited in thin layers on a porous ceramic support tube (PST). However, PST cells, although sufficiently porous, present inherent impedance to airflow toward the cathode.
Fig.3. Tubular fuel cell based on a porous support tube
The monolithic design is composed of thin cell components in a compact corrugated structure. There are two types of configuration: co-flow and cross-flow ( Fig.4 shows a co-flow design). A disadvantage of these designs is the difficulty in fabricating the corrugated structure. It has to be made by co-firing which depends on firing shrinkage (thermal expansion) matching of the cell components.
Fig.4. Co-flow configuration test cell
Applications
Although the first fuel cell was developed in the 1800s, the technology was not taken up until the 1960s by NASA who sought a safe and reliable power source for manned missions. It funded more than 200 research projects on the technology and fuel cells have now powered projects from Apollo to the Space Shuttle.
More recently, the automotive and power generation industries have led the research; however, there are several interesting applications in the consumer goods sector.
Automotive
The US Department of Energy predicts that if 10% of cars in the US were powered by fuel cells, regulated air pollutants would be cut by a million tons per year and carbon dioxide by 60 million tons.
Recently, several major manufacturers have announced fuel cell programmes with field tests due for completion by 2002. End of decade sales are expected to exceed €300 million and analysts predict that by 2020 fuel cells will power most new vehicles.
The SPFC offers potential range and performance to match the internal combustion engine and the development of commercially competitive systems appears increasingly probable. The AFC may find commercial opportunities as a range extender for battery powered buses. Many organisations are now working on different versions of DMFC and more recently on direct ethanol fuel cells (DEFC) for vehicles. These would operate at low temperatures, making them attractive for intermittent vehicle use. The intermediate temperature SOFCs are very long-term candidates for applications in public transport.
Power generation
The prospects appear more attractive in the distributed power generation and combined heat and power markets, and the availability of competitive fuel cell systems could enable new markets in these sectors, particularly at the very small scale. The low temperature fuel cells (SPFC and PAFC) seem to offer the best prospects in the short to medium term. PAFC systems are expected to play a significant role where electricity costs are high and dispersed generation is preferred. The SPFC could be competitive though further development would be required in the short term.
In the longer term, the high temperature fuel cells, particularly the tubular SOFC system is likely to be available within the next five years, offering the prospect for dramatically improved electrical generation efficiency when integrated with a gas turbine.
Consumer goods
On a completely different scale, researchers have developed a fuel cell with a volume of approximately 5mm
3. It is envisiged that this type of device will be used in mobile hardware such as phones, where half the weight is usually attributable to the battery.
The companies developing fuel cells for portable electronics applications have different motivations. They are interested in longer operating times and a refuelling time of seconds. According to manufacturers, fuel cells would be able to power a mobile telephone for 30 days or keep a laptop running for 20 hours.
A vacuum cleaner being developed in the US will run on a hydrogen fuel cell weighing less than 500g. Apart from its green credentials, the main advantage of the cleaner is that it will not need to be plugged into the mains. Gardening tools and DIY power tools are other prime candidates for fuel cell technology.
Current issues
Fuel remains one of the biggest problems in fuel cell acceptance and implementation. This is due to increasing concern regarding the costs of producing, distributing and storing hydrogen.
However, since the late 1990s, there has been significant progress in fuel cells that convert hydrocarbon fuels directly to electricity, direct methanol fuel cells and solid oxide fuel cells for example. Methanol and ethanol are emerging as the renewable fuels of most commercial interest.
Conclusions
Fuel cell technologies have the capacity to replace combustion engines in every niche they occupy, conventional batteries in portable power and gas turbines in power generation. There is a clear market for the functional advantages that fuel cells could provide, such as high efficiency, low emissions, low noise and modularity. Demand for these features is only likely to increase and the commercial arrival of fuel cells is almost guaranteed.
